Radio frequency coaxial switches



Dec. 3, 1968 W, G, BURT, JR 3,414,849

RADIO FREQUENCY COAXIAL SWITCHES Filed May 16, 1966 2 Sheets-Sheet l j. \1 3 z afi-fi? s N i fier. J 37 38u-S I l l 125 l i? Y, t 3' n ljxinza i* ,i i o 23 Y ii, 13

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RADIO FREQUENCY COAXIAL SWITCHES Filed May 16, 196e v K 2 sheets-sheerz l l I B3 I e2 V 86 l' 'l 8| a 7 a 'l 5| f 4 88 i United States Patent O 3,414,849 RADIO FREQUENCY COAXIAL SWITCHES William G. Burt, Jr., Weston, Mass., assignor to Microwave Associates, Inc., Burlington, Mass., a corporation of Delaware Filed May 16, 1966, Ser. No. 550,242 9 Claims. (Cl. 333-97) ABSTRACT OF THE DISCLOSURE This invention relates to coaxial switches for radio frequency applications. In particular it concerns coaxial switches using double-groundplane strip internal connections. In accordance with the invention, the center conductor of a double-ground-plane line connects selected input and output connections together when it is moved to the normal mid position between the two ground planes. For disconnection the center conductor is moved to a position in uniform electrical contact with one of the ground planes. In the disconnected position, the two ground planes form a waveguide below cutoff.

Background of the invention RF coaxial switches are used for transmit-receive switches to switch a single antenna between transmitter and receiver and for many transfer purposes. As the frequencies of interest have climbed ever higher up to thousands of megacycles it has been increasingly diiiicult to maintain both good isolation and low VSWR (voltage standing wave ratio). While electronic devices such as semiconductor diodes, ionizable electron tubes and various breakdown devices have been used in rectangular waveguide and to a lesser extent in coaxial waveguide, mechanical switches are still preferable in many instances. One of the difficulties with mechanical switches has been the need to move the switching element a considerable distance to prevent undesired pickup. Another ditliculty has been the need of a double action including short circuiting of the disconnected line to `assure isolation.

An example of a prior art switch that bears a resemblance to the present invention will be found in Atwood et al., U.S. Patent 2,498,907. Due to an utter lack of dimensional data or any suggestion of theoretical aspects, many of the probable distinctions between Atwood et al. Iand the present invention would be argumentative. Some specific and significant distinctions are nonetheless quite apparent. The present invention uses a double-groundplane line and Atwood et al. have a bad discontinuity caused by the recess in wall 13 (FIG. 2 of Atwood et aL). This wall occupies the analogous position to one of the ground planes in the present invention and such -a discontinuity Would increase the VSWR. Further, bars 40 and 41 of Atwood et al. in the grounded position are supported by contacts 60, 61 and 62 so that (as illustrated in FIG. 1 of Atwood et al.) they have considerable portions that are ungrounded. These portions would exhibit frequency sensitive reactive effects. As illustrated, it appears that the Atwood et al. switch requires greater mechanical movement then is required of the present invention as will be described. Full details of double-groundplane lines as used herein will be found in Microwave Engineering by A. F. Harvey, Academic Press, 1963, pages 408 to 411.

Switches constructed in strip-line are briefly referred to on pages 418 and 419 of the same book. U.S. Patent 2,997,669 to Charles describes a switch having tri-plane transmission paths (column 4, lines 41 and 42). The switch described therein uses a moving shutter (100) to provide isolation both by shorting and shielding the connection terminals. This extra moving element is avoided in the present invention wherein the switched path inherently becomes a waveguide beyond cutoff in the disconnected condition.

Description of the invention It is an object of the invention to define a coaxial RF switch operating as a double-ground-plane line when closed `and as -a waveguide beyond cutoff when open.

It is a further object of the invention to define a novel coaxial RF switch of the single pole double throw type.

It is a further object of the invention to define a novel coaxial RF transfer switch.

It is still a further object of the invention to define coaxial RF switches of `small mechanical movement.

Further objects and features of the present invention will become apparent upon treading the following specification together with the drawings in which:

FIG. 1 is an elevation in section of a single pole double throw RF coaxial switch in accordance with the vinvention;I

FIG. 2 is a cross-sectional view taken through line 2-2 of FIG. 1;

FIG. 3 is a plan view of the switch elements at the common terminal of the FIG. 1 switch;

FIG. 4 is a cross-sectional view of double-ground-plane line as used for the switch elements of the present invention;

FIG. 5 is an elevation in section of an RF transfer switch in accordance with a second embodiment of the invention; and,

FIG. 6 is a plan View of the switching elements of the switch of FIG. 5 as viewed from line 6--6 of FIG. 5.

Switches in accordance with the present invention are referred to herein as coaxial switches, however the switch paths are double-ground-plane lines and the input con- F nections can be in the form of coaxial lines, parallel plate lines, waveguide or any other acceptable transmission line for RF energy. The term coaxial is used as a generic term for the type of switch involved recognizing the fact that parallel plate lines are an evolution of concentric coaxial lines, see Microwave Engineering, page 407.

Referring now to FIG. 1 a single pole double throw coaxial switch is depicted having a support base 10, a dust cover 11, and three coaxial connectors 12, 13 and 14. As will be seen, connector 13 is the common connector which is connected alternatively to one of connectors 12 and 14 by the switch. Connectors 12, 13 and 14 are mounted in the conventional manner to base 10 with their respective central terminals 15, 16 and 17 passing through apertures 20, 21 and 22 and projecting into a common cavity 23. Cavity 23 is formed by base 10 and cap 25 mounted securely on top of base 10 as by bolts 26. Cavity 23 is bounded by micro-finished surfaces 27 of base 10 and 28 of cap 25. These surfaces 27 and 28 are parallel and form two ground plane surfaces of a doubleground-plane line. Typically they may be spaced about 1A; inch apart.

Positioned in cavity 23 is a iirst movable conductive strip element 30 arranged so that it can make a connection between terminals 15 and 16. A second movable conductive strip element 31 is positioned so that it can make electrical contact between terminals 16 and 17. As depicted in FIG. 1, element 31 is in the noncontacting or open position resting in uniform electrical contact against surface 28. Elements 30 and 31 are mounted respectively on pins 32 and 33. These pins each have an axis perpendicular to the plane of the respective element. One end of each of pins 32 and 33 rides in a respective recess in base 10 with pin 32 resting against a compression spring 35 and pin 33 resting against a similar cornpression spring 36. The opposite ends of pins 32 and 33 each pass through apertures in cap 25 so that they can 'be' movably'actuated by' a'v ro'cke'i arm 37. Rocker arm 37 pivots upon a centrally located axle 38 and is normally positioned under control of compression spring 40 so 'that a rst end 41 of the rocker arm pushes against pin 33.- A second end 42 of the rocker arm is positioned to contact the end of pin 32. A solenoid actuator 43 suitably mounted on cap `25 has an actuating member 45' in contact with ya projection 46 of rocker arm 37.

FIG. 2 is a cross-sectional view through line 2 2 of FIG. 1 showing connector 13 (not in section) with central terminal 16, surface 27 of ybase 10, surface 28 of cap V25, element 31 in the open position and element 30 in the closed position. Axle 38 mounted in cap 25 supports rocker arm 37 for rotation as actuated by solenoid 43. fr

FIG. 3 depicts the ends of elements 30 and 31 as they contact terminal v16. As can be seen in the figure the ends of these elements are cut at a slant so as to meet with supplementary angles. This permits them to pass each other lduring switching operation and still make good area contact on terminal 16. While these slanted ends have been found desirable it will be understood that `they are not critical to the inventive concepts.

In operation, rocker arm 37 normally is forced against pin 33 at end 41 by the tension of spring 40. Tension of spring 40 is desirably equal to or greater than twice the tension of spring 36 so that the average spring pressure operative in closing switch element 31 is as great as the spring pressure operative in opening switch element 31. It will be seen that thetension of spring 36 is considered by itself in opening element 31 and must be subtracted from the tension of spring 40 when switch element 31 is closed. Thus with rocker arm 37 in its normal position switch element 31 will contact terminals 16 and 17 providing a conductive path from connector 13 to connector 14. For throwing the switch to its opposite position, solenoid 43 is energized so that member 45 pushes against the tension of spring 40 with end 42 of the rocker. arm forcing pin 32 down against spring 35. This pushes element 30 into position contacting terminals and 16 and providing a conductive path therebetween. At the same time the other end 41 of rocker arm 37 releasespin 33 so that element 31 is forced up into contact with surface 28 by spring 36.

The rotational force applied to arm 37 by solenoid 43 should be equal to or greater than about twice the rotational force applied by the tension of spring 40. It will be understood that leverage, i.e. the radial distance between axle 38 and the point at which pressure is applied to rocker arm 37, must be taken into account.

It -is critical in the present invention that surface 28 of cap 25 an-d the surfaces of elements 30 and 31 when pushed in contact with surface 28 in the open position should `mate with each other uniformly and precisely. Switches made in accordance with the invention have had these surfaces machined to a precise finish and flatness. A 64 micro-inch finish together with end to end flatness within .002 inch has been found satisfactory. With machining of this precision a minimum isolation of 60 decibels has been maintained from 1 gigacycle to 1l gigacycles.

FIG. 4 illustrates in cross-section the cavity formed by surfaces 27 and 28 with one of the switch elements, for example, 30 positioned in the closed switch position. The dimensional limitations on these elements are generally those described for parallel plate transmission line systems as described in Microwave Engineering, supra. As will be seen by reference to that publication, the width w of the surfaces 27 and 28 bounding the cavity should be large with relation to the width a of switch element 30. These dimensions w and a as well as the other dimensions, for example the thickness t of switch element 30 and the spacing d between surfaces 27 and 28, will determine the impedance characteristics of the double-ground-plane line. For convenience they are preferably selected so that this impedance will match the impedance of the lines being switched. In a practical application VSWR values of 1.3:1 or less over the frequency range of l gigacycle to l1 gigacycles have been thus achieved.

While element 30 has been described as positioned at the Imid-point between surfaces 27 and 28 when in the closed position, this is only to obtain a symmetrically balanced line. For an unbalanced line it may be displaced one way or the other.

The particular embodiment depicted in FIGS. l and 2, bounds cavity 23 on the sides and ends as well as top and bottom. Conductive sides spaced less than one free space wavelength apart at the maximum propagating frequencies of the switch help suppress propagation of extraneous modes in the open switch condition. While this has also been found desirable from the standpoint of keeping the cavity free of foreign matter `and from the point of view of simple mechanical structure, it will be understood as depicted in FIG. 4, double-groundplane line does not require closure at the sides. Exemplary `dimensions of a commercial embodiment are given beloW.

EXAMPLE I Referring to FIG. 4 exemplary dimensions of the cavity and switch element are as follows:

Inches t .O25 d .125

It has been `found preferable, from the standpoint of speed, compactness and effective cutoff isolation high into the gigacycle region, to keep the movement of the switching element in the range of .050 inch and less.

A four terminal transfer switch in accordance with the invention is illustrated in FIG. 5. The switch has a base 50, a dust cover 51, two connectors 52 and 53 showing in the figure and two other connectors (not shown) behind them. As in FIG. l connectors 52 and 53 have respective terminals 55 and 56 extending through apertures 60 and 61 in base 50 and extending into cavity `63. This cavity is formed between base 50 and cap 65 mounted upon base 50. Surfaces 67 of base 50 and 68 of cap 65 bound cavity 63. Conductive switch element 70` is supported in cavity 63 by pin 72. An end view of two other conductive switch elements 73 and 74 depicted in the open position are shown opposite terminals 55 and 56. Elements 73 and 74 are supported on pins 75 and 76, respectively, which are shown only in part. Each of pins 72, 75 and 76, as well as a fourth pin (see FIG. 6) directly behind pin 72 are positioned in recesses in base 50 upon compression springs similar to spring 77 supporting pin 72. The opposite end of each pin passes freely through an aperture in cap 65. Rocker arm 78 operates to alternatively depress element 70 or 74. A second rocker arm 80 alternatively depresses either switch element 73 or switch element 71 (seen in FIG. 6) on the back side of the switch.

One end of rocker arm 78 is normally depressed by compression spring 81. One end of rocker arm 80 is normally depressed by compression spring 82. Springs 81 and 82 are supported to bear against an end of the respective rocker arm by plate 83. Plate 83 also supports a solenoid actuator 85 operating a plunger 86. Plunger 86 has radially extending arms `87 connecting with the ends of rocker arms 78 and 80 opposite the spring loaded ends. The arms 87 are a single element rockably affixed to the end of plunger 86 so that the ends of the arms can bear with equal force on both rocker arms 78 and 80.

Rocker arms 78 and 80 are mounted to rock on pivots 88, only one of which is shown in FIG. 5. Pivots 88 are suitably mounted on fixed supports (not shown) secured to cap 65.

FIG. 6 shows the arrangement of switch elements 70, 73 and 74 as well as 71 (not shown in FIG. 5) in plan view. The switch elements are arranged in a rectangle with a terminal of one of the four connectors 52, 53, 92 and 93 (the latter two not shown in FIG. 5) located at each corner of the rectangle. As in the case of FIG. 3, the ends of the switch elements are shaped to provide good contact with the respective terminals while still providing clearance to mechanically bypass each other. To prevent any rotation of the switch elements as they are moved, insulating antirotation pins 90 are positioned as stops adjacent to the ends of the elements. Lines 95 paralleling the outside of the switch elements and lines 96 paralleling the inside of the switch elements define the width of the cavity containing the elements.

Referring back to FIG. 5 and as in FIG. l, it is to be noted that the switch elements in the open position are pressed into uniform precision electrical contact against surface 68.

In operation, elements 73 and 74 are normally closed under pressure from springs 81 and 82. Actuation of solenoid 85 pushes arms 87 on plunger 86 against the opposite ends of rocker arms 78 and 80 causing the switch elements to reverse positions. Clearance between the end 41 of the rocker arm 37 and the confronting end of pin 33 should be about 0.010" when element 31 is in the open position, in order to assure uniform grounding of contact 31 in that position. Similar clearance should be present between rocker arm end 42 and pin 32 when switch 30 is in the open position, for the same reason.

While the invention has been described in relation to three terminal and four terminal specific embodiments, it will be understood that these are only exemplary and that the inventive principles can be applied to many variations of the specific disclosed embodiments with any desired terminal configurations within the scope of the appended claims.

I claim:

1. An RF coaxial switch for frequencies up to at least l1 gigacycles having a mechanical switching element comprising:

(a) an electrically conductive first member having a flat surface;

(b) an electrically conductive second member having a fiat surface;

(c) means to support said second member so that its flat surface is parallel to and spaced from the iiat surface of said first member a distance less than onehalf of the free space wavelength of any frequency to be propagated through said switch so as to form a cavity therebetween;

(d) an electrically conductive third member having two fiat surfaces and a width that is small compared to the width of said cavity;

(e) movable means to support said third member between said rst member and said second member with said two flat surfaces parallel to and facing the respective flat surfaces of said rst member and said second member;

(f) a plurality of spaced R-F connectors each having a terminal projecting through the surface of said first member; and,

(g) means to move said movable means whereby said third member can be selectively moved between a first position in uniform electrical contact throughout its entire length against the flat surface of said second member and a second position between and spaced from the surfaces of said first member and said second member so as to contact at least two of said terminals providing a conductive path therebetween.

2. An RF coaxial switch for frequencies up to at least l1 gigacycles according to claim 1 in which said third member moves in the range of .050 inch or less in moving between said first position and said second position.

3. Au RF coaxial switch for frequencies up to at least 11 gigacycles according to claim 1 in which said switch is a single-pole double-throw switch having three of said terminals, and in which one of said three is a common connector and said third member comprises a first switch element and a second switch element movable by a single rocker arm so that one position of said arm places said first switch element in contact with both the terminal of said common connector and a first of the remaining two terminals while said second switch element is placed in uniform contact with the surface of said second member, and the other position of said arm places said second switch element in contact with both the terminal of said common connector and the second of the remaining two terminals while said first switch element is placed in uniform contact with the surface of said second member.

4. An RF coaxial switch for frequencies up to at least 1l gigacycles according to claim 1 in which the flat surface of said second member and the fiat surface of said third member facing the flat surface of said second member are each precisely shaped to an end-to-end flatness within .002 inch tolerance to provide said uniform electrical contact.

5. An RF coaxial switch for frequencies up to at least l1 gigacycles according to claim 1 in which said switch is a four terminal transfer switch, said plurality is four `connectors positioned in a rectangle and said third member is four switch elements forming two parallel pairs each element having ends terminating at two corners of said rectangle for contacting the terminals of said connectors.

6. An RF coaxial switch for frequencies up to at least 11 gigacycles according to claim 5 in which said pairs are operated alternatively whereby when one pair is dcpressed to contact the four connector terminals the other pair is placed in uniform contact with the surface of said second member and vice-versa.

7. An RF coaxial switch for frequencies up to at least 1l gigacycles according to claim 6 in which said four switch elements are moved by two spring-loaded rocker arms actuated against the spring-loading by a single solenoid device.

8. An RF coaxial switch for frequencies up to at least l1 gigacycles comprising means providing a section of wave guide, an elongated conductive element in said wave guide, means to locate said element selectively in one of two positions, the first of which positions locates said element in uniform electrical contact with a wall of said wave guide and the second of which positions locates said element within said wave guide out of contact with any of the walls thereof, and within 04.05 inch of its first position, input and output conductive means extending respectively into said Wave guide through the walls thereof and terminating at spaced-apart points within said wave guide, said elongated element in said second position contacting said input and output means at said points and thereby bridging them, said wave guide functioning as a coaxial conductor between said input and output means when said elongated element is in said second position and functioning as a wave guide beyond cut off frequencies in the operating range of the switch when said elongated element is in said first position.

9. A high frequency single pole, double throw RF coaxial switch comprising means providing a section of wage guide, an input terminal positioned in said wave guide, and two output terminals positioned in said wave guide and spaced from said input terminal, two elongated 7 conductive elements in said wave guide, each associated with one of said output terminals, and each element positionable selectively in one of two positions, the first of which positions locates said element in uniform contact with, and effectively grounded substantially through its entire length to a wall of said wave guide and the second of which positions locates said element within said wave guide out of contact with any wallthereof and in conductive contact with said input terminal and one of said output terminals, thereby bridging said input ter- Iminal and said output terminal, means to selectively and `cooperatively move the rst of said elongated conductive members intosaid rst position ,andv the second conductor simultaneously into said second posi- 8 second position and said second .conductor simultaneously into said rst position, said wave guide functioning as a coaxial conductor between said input and output lmeans when said elongated element is in said second position and functioning as a wave guide beyond cut 01T when said elongated element is in saidl rst position. l

References Cited.

UNITEDV STATES PATENTS 2,662,142 12/1953 Nelson 333-97 2,498,907 v 2/1950 Atwood et al 333-97 HERMAN KARL SAALBAC'H, Primary Examiner.

tion, and selectively to move said rst conductor into said l5 L. ALLAHUT, AssstantrExamner. 

